PA system on Shabbos even transistorized

 Igros Moshe (OC III #55) Even though there are a number of advantages of a transistorized pa system in regard to Shabbos prohibitions, nevertheless it is still prohibited to use either on Shabbos or Yom Tov.  There are four reasons for prohibiting a pa system. 1) Even if it is setup prior to Shabbos it is still prohibited as we see that grain can not be placed in a mill before Shabbos to be ground on Shabbos since it makes noise on Shabbos similar a clock can not be set up to sound the time on Shabbos. 2) Speaking into a pa system causes greater use of electricity which might be prohibited even though the halachic status of electricity has not been clearly decided  3) It is possibly included in the rabbinic decree against use of musical instrument on Shabbos because you might make adjustments. This is very common that the system needs to be adjusted when someone is speaking and this can be done by almost everyone as opposed to a musical instrument that requires an expert. 4) It is possible that the sound that comes from a pa system is not considered the original sound spoken into the microphone. The creation of a new sound is probably a prohibited act that might be a Torah prohibition but I don’t have time to elaborate. Three of the four reasons apply also to a transistorized pa system with the third reason  possibly not applying. Thus it is prohibited and one should not be lenient.